Electric discharge lamp

ABSTRACT

The present invention includes an electrode having a filament with the end of each lead wire connected to each end of the filament, a cylindrical glass tube in which the electrode is provided, and a thermal fuse which is electrically connected to the lead wires in series, and is provided near the electrode on an outer peripheral surface of the glass tube, wherein, when a position on the outer peripheral surface of the glass tube which extends in a radial direction of the glass tube from a connecting portion of the filament and the lead wire, serve as a reference position, in case where the diameter of the glass tube is 8 mm or more, the thermal fuse is arranged at a distance of 1.5 mm or less from the reference position in the longitudinal direction of the glass tube.

This application is based upon and claims the benefit of priority from Japanese patent application No. 2006-191980, filed on Jul. 12, 2006, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric discharge lamp, such as a fluorescent lamp, having an electrode with a filament and lead wires connected thereto.

2. Description of the Related Art

Generally, a hot cathode electric discharge lamp includes an electrode and a cylindrical glass tube which has electrodes at each end inside the glass tube. An electric discharge lamp, comprising an electrode having a filament with the end of each lead wire connected to each end of the filament. The other ends of the lead wires of the electrode are supported by a stem formed in an end portion of the glass tube.

In this kind of hot cathode electric discharge lamp, since anomalous discharge is caused after an emissive material (emitter) is dispersed at end of life, a preheating current increases, or an electrode material, a lead wire material, or the like disperses and adheres on a surface of a stem, which supports lead wires because of insufficient discharge. For this reason, the electric discharge lamp generates heat by a short circuit arising between lead wires and a preheating current flowing on the surface of the stem, and has a possibility of melting the glass material, which makes up the stem, and the glass tube.

As a measure of preventing the stem and the like from melting by such an anomalous discharge of a hot cathode electric discharge lamp, Japanese Patent Laid-Open No. 11-203934 discloses a construction using a thermal fuse which shuts supply off current at the time of anomalous discharge. This patent publication discloses the construction of a thermal fuse that is arranged on an outer peripheral surface of the glass tube near an electrode.

According to the above-mentioned temperature rise of the glass tube at the time of the anomalous discharge, the tendency of the temperature rise is that the temperature rises faster at the position near the electrode and rises slower at positions away from the electrode in a longitudinal direction of the glass tube. For this reason, there is an issue that dispersion of temperature which the thermal fuse detects becomes large according to the installation position of the thermal fuse.

Nevertheless, in the technology disclosed in Japanese Patent Laid-Open No. 11-203934 mentioned above, the position where the thermal fuse is arranged to the electrode is ambiguous. Thus, this patent publication performs neither disclosure nor suggestion at all about the relative position between the thermal fuse and the electrode, and the relationship between the diameter of the glass tube and the arrangement of the thermal fuse. Hence, the construction of this patent publication only discloses a possibility that the dispersion in operating temperature of the thermal fuse may become large because of the position where the thermal fuse is arranged.

Then, there is the need to avoid a relatively high temperature in an electric discharge lamp when the lamp is used as a light source to provide back light for a transmissive liquid crystal display. Such a construction is inconvenient in that dispersion in the operating temperature of a thermal fuse adversely affect a member which is used to construct a liquid crystal display even if the stem does not reach melting point.

SUMMARY OF THE INVENTION

The present invention aims at providing an electric discharge lamp which can reduce dispersion in operating temperature of a thermal fuse at the time that emissive material of an electrode is exhausted and at the time an anomalous discharge is caused, and can stop supply of a current before the temperature of a part of a glass tube becomes relatively high.

In order to achieve the above-described object, the electric discharge lamp according to the present invention includes an electrode having a filament with the end of each lead wire connected to each end of the filament, a cylindrical glass tube in which the electrode is provided, and a thermal fuse which is electrically connected to the lead wires in series, and is provided near the electrode on an outer peripheral surface of the glass tube, wherein, when a position on the outer peripheral surface of the glass tube, which extends in a radial direction of the glass tube from a connecting portion of the filament and the lead wire, serves as a reference position, in case where the diameter of the glass tube is 8 mm or more, the thermal fuse is arranged at a distance of 1.5 mm or less from the reference position in the longitudinal direction of the glass tube.

In the electric discharge lamp according to the present invention which is constructed as described above, when anomalous discharge arises, dispersion of operating temperature of the thermal fuse provided on the outer peripheral surface of the glass tube is suppressed, and supply of power is shut off well by a thermal fuse before the temperature of a part of the glass tube becomes relatively high.

An exemplary advantage according to the present invention is that, it is possible to reduce dispersion in operating temperature of a thermal fuse at the time when an anomalous discharge occurs, and to stop supply of a current before temperature of a part of a glass tube becomes relatively high.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram illustrating a fluorescent lamp of this exemplary embodiment; and

FIG. 2 is a graph for describing the relationship between the temperature of each section of a glass tube, and the lighting elapsed time in the above-mentioned fluorescent lamp.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

Hereafter, specific exemplary embodiments will be described with reference to drawings.

As illustrated in FIG. 1, fluorescent lamp 1 as an electric discharge lamp includes electrode 6, and cylindrical glass tube 5 which has electrodes 6 at each end inside the glass tube.

Electrode 6 has filament 7 with the end of each lead wires 8 connected to each end of the filament. The other ends of lead wires 8 of electrode 6 are supported by stem 15 formed in an end portion of glass tube 5. In addition, exhaust pipe 16 communicating inside glass tube 5 is provided in stem 15.

In addition, on an outer peripheral surface of glass tube 5, thermal fuse 12 is provided near electrode 6. This thermal fuse 12 is electrically connected to lead wires 8 in series.

Then, when a position on the outer peripheral surface of glass tube 5, which extends in a radial direction of glass tube 5 from a connecting portion 11 of filament 7 and lead wire 8, serves as a reference position P1, in cases where the diameter of glass tube 5 is 8 mm or more, thermal fuse 12 (strictly speaking, the center of thermal fuse 12 in the longitudinal direction) is arranged at a distance of 1.5 mm or less from the reference position P1 in the longitudinal direction of the glass tube 5.

In addition, although FIG. 1 illustrates only thermal fuse 12 arranged in one end side of glass tube 5, thermal fuse 12 is also arranged similarly in the other end side of glass tube 5.

Although described later, when the position of thermal fuse 12 is 1.5 mm beyond reference position P1, when glass tube 5 with 8 mm of diameter is used, dispersion in operating temperature based on the position of thermal fuse 12, gradually becomes large, and hence, this position of thermal fuse 12 is not preferable. In addition, when the diameter of glass tube 5 is 8 mm or more, dispersion in operating temperature of thermal fuse 12 becomes large according to the amount of clearance from reference position P1. For this reason, it is preferable that thermal fuse 12 is arranged in a region in which the amount of clearance from reference position P1 is 1.5 mm or less.

In addition, when the diameter of the glass tube is smaller than 8 mm, even if the amount of clearance from reference position P1 exceeds 1.5 mm, the dispersion in the operating temperature of the thermal fuse is relatively small. For this reason, the thermal fuse may be located 1.5 mm further from reference position P1.

Regarding fluorescent lamp 1 constructed as described above, a temperature change near electrode 6 of glass tube 5 will be described.

FIG. 2 illustrates the temperature change near electrode 6 on the outer peripheral surface of glass tube 5 in fluorescent lamp 1.

At the time that the anomalous discharge of fluorescent lamp 1 occurred, the temperature rise caused by anomalous discharge on the outer peripheral surface of glass tube 5, at the time when an emissive material (emitter) of electrode 6 disappears, was measured. In addition, a glass tube having a diameter of 8 mm was used as glass tube 5. FIG. 2 illustrates measurement results of the temperature rise following the lighting elapsed time at the time of the anomalous discharge in each of the positions that are located away from reference position P1 by 1.5 mm, 3 mm, and 5 mm.

As illustrated in FIG. 2, the temperature rise was fastest at reference position P1 that corresponds to filament connecting portion 11, but the temperature rise was slow away from reference position P1.

When six seconds elapsed after the anomalous discharge which caused the temperature at reference position P1 to reach 100° C., the temperature at a position which was 5 mm away from reference position P1 only rose 30° C., from 30° C. to 60° C. For this reason, when thermal fuse 12 which detects the temperature rise of 100° C. is used at this location which is 5 mm away from reference position P1, temperature at reference position P1 will rise to about 250° C.

Similarly, when temperature at the location which is 1.5 mm away from reference position P1 reaches at 100° C., the temperature in reference position P1 will rise to about 120° C. In addition, when temperature at the location which is 3 mm away from reference position P1 reaches at 100° C., the temperature in reference position P1 will rise to about 150° C.

Hence, in the vicinity of electrode 6 on the outer peripheral surface of glass tube 5, the temperature rise is greatly different according to the amount of clearance from reference position P1, and hence, it is necessary to specify a detecting position at which temperature is detected by thermal fuse 12.

For this reason, by the arrangement of thermal fuse 12 that is provided within a range of 1.5 mm or less from reference position P1, in the longitudinal direction of glass tube 5, and that is installed on the outer peripheral surface of glass tube 5, the dispersion in the temperature which thermal fuse 12 detects, becomes small. Hence, the partial temperature rise of glass tube 5 caused by the anomalous discharge of electrode 6 is prevented.

As mentioned above, in fluorescent lamp 1, when the diameter of glass tube 5 is 8 mm or more, thermal fuse 12 is positioned away from reference position P1, which corresponds to filament connecting portion 11, by a distance of 1.5 mm or less, in the longitudinal direction of glass tube 5. Because of this construction, it becomes possible to stop supply of a current before the temperature of a part of glass tube 5 becomes high after exhausting the emitter (emissive material) of electrode 6, which is a hot cathode, and after beginning anomalous discharge. In consequence, it is possible to reduce the dispersion in the operating temperature at which the supply of current is stopped.

In addition, the electric discharge lamp according to the present invention is suitable for use as a light source for back lighting to illuminate a transmissive liquid crystal display panel in which it is necessary to prevent the temperature from becoming relatively high. Even if the stem does not melt, the present invention prevents dispersion in the operating temperature of a thermal fuse that adversely affects a member which is used in the construction of a liquid crystal display panel.

While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those ordinary skilled in the art that various changes in form and details may be made there in without departing from the spirit and scope of the present invention as defined by the claims. 

1. An electric discharge lamp, comprising an electrode having a filament with the end of each lead wire connected to each end of the filament, a cylindrical glass tube in which the electrode is provided, and a thermal fuse which is electrically connected to the lead wire in series and is provided near the electrode on an outer peripheral surface of the glass tube, wherein, when a position on the outer peripheral surface of the glass tube which extends in a radial direction of the glass tube from a connecting portion of the filament and the lead wire, serve as a reference position, in case where the diameter of the glass tube is 8 mm or more, the thermal fuse is arranged at a distance of 1.5 mm or less from the reference position in the longitudinal direction of the glass tube.
 2. The electric discharge lamp according to claim 1, wherein the electric discharge lamp is a light source which illuminates a transmissive liquid crystal display panel from a back side of the liquid crystal display panel. 